The field of the disclosure is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the disclosure relates to systems and methods for tracking and controlling artifacts caused by motion during a MRI procedure.
MRI uses the nuclear magnetic resonance (“NMR”) phenomenon to produce images. When a substance such as human tissue is subjected to a uniform magnetic field (“main magnetic field”), B0, the individual magnetic moments of the nuclei in the tissue attempt to align with this magnetic field, but precess about it in random order at their characteristic Larmor frequency, ω. If the substance, or tissue, is subjected to an excitation magnetic field, B1, that is in the plane transverse to the main magnetic field, B0, and that is near the Larmor frequency, ω, the net aligned magnetic moment of the nuclei may be rotated, or “tipped,” into the transverse plane to produce a net transverse magnetic moment. A signal is emitted by the excited nuclei, or “spins,” after the excitation magnetic field, B1, is terminated. The emitted signal may be received and processed to form an image.
When utilizing these emitted “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically-proven pulse sequences and they also enable the development of new pulse sequences.
Depending on the technique used, many MR scans currently require many minutes to acquire the necessary data used to produce medical images. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughout, improves patient comfort, and improves image quality by reducing motion artifacts. Many different strategies have been developed to shorten the scan time.
For example, one popular category of pulse sequences are so-called gradient echo sequences. Within this category, the spoiled gradient echo and three-dimensional spoiled gradient-recalled echo (SPGR) or fast low angle shoot (FLASH) pulse sequences are often used in neuroimaging applications. Specifically, the SPGR or FLASH sequence forms the basis of many 3D neuroimaging sequences, but acquisition times often stretch to several minutes. Lengthy acquisitions in neuroimaging applications can be particularly troublesome because it is imperative that the subject remain motionless during the duration of the sequence. That is, in neuroimaging applications, motion can be particularly damaging to the resulting images because of the complexity of the structures being imaged and studied in neuroimaging applications.
Motion-correction systems in MRI can be grouped into two general methods: prospective and retrospective. Retrospective methods use information about the subject's motion to estimate what k-space data would have been measured if the subject had not moved during scanning. Prospective methods use motion-tracking data acquired during the scan to follow the subject with the gradient axes of the sequence, measuring the desired k-space data directly. Additionally, it is possible to combine the two methods so that retrospective processing corrects residual errors in the prospective system. A retrospective system can access all of the k-space data while performing reconstruction; a prospective system must necessarily rely only on previous measurements to estimate the current position of the patient. However, a prospective system avoids the need to estimate missing k-space data, allowing for direct reconstruction while avoiding possible sources of estimation error in the k-space data.
Also, one can differentiate between two types of motion correction problems that arise in MRI: between-scan motion and within-scan motion. For between-scan motion, several retrospective motion correction methods are available that register either slice-by-slice or volume-by-volume to estimate the data that would have been acquired in each volume if the subject had not moved. Prospective motion correction can also be employed for this problem, such as the orbital navigator system that inserts 3-plane circular k-space navigators, or the PACE system that registers each completed EPI volume back to the first time-point and so requires no navigators.
For in-scan motion, several methods are available, such as PROPELLER and the like that use redundant sampling of the center of k-space during each repetition time (TR) and estimate motion-free k-space data retrospectively. Also, prospective motion correction is available, such as by using cloverleaf navigators. The use of cloverleaf navigators is useful with SPGR/FLASH sequences.
However, in order to maximize SNR/time efficiency, short TR protocols are often used in neuroimaging applications. Such short-TR protocols, by definition, have very-little dead time and, thus, force navigators to be very short and provide limited k-space coverage. That is, as the TR is reduced, the effectiveness of the navigator is reduced because there is less information gathered by the navigator to use to form an estimate the subject's head motion.
It would therefore be desirable to provide a system and method for controlling the competing constraints of neuroimaging applications that desire extended acquisition times and the need to control or compensate or correct for patient motion during such acquisitions.